27Sep 2014

Today’s post takes us on a brief journey inside a lithium-ion battery. What is its internal structure and how does it actually store electrical energy?

Let’s first start with a capacitor…the kind of stuff you studied in high school physics. It has two plates separated by an insulator, for example, an air gap, or may be a thin sheet of mica. If you recall your science experiments with a capacitor, it stored electrical energy because electric charge (electrons here) was pulled from one plate, traveled across an electric circuit and moved the opposing plate. This act of separating electric charge in the basic mechanism of energy storage. Its evidence in an electric circuit is the presence of a voltage across the two plates (or terminals) of the capacitor.

So why can’t we use an electric capacitor for energy storage? Simple…its energy density is way too small. But wait, you say, capacitors in our high-school physics classes were never described in terms of energy density. They were described in terms of capacitance measured in units of Farads.  No problem, we can use either methodology. Typical capacitors have capacitances ranging from a few picofarads (one pico is one trillionth) to may be millifarads (one milli is one thousandth). By comparison, a lithium-ion battery has an equivalent capacitance about one million times larger. 

So a rechargeable battery is fundamentally an electrical device for storing energy at the highest possible energy density — or in very simplistic terms, think of it as a capacitor whose two plates are separated from each other by only nanometers (one nano is one billionth). 

The internal structure of a lithium-ion battery is remarkably yet deceivingly simple. Much as a capacitor, it has two metal plates called electrodes. In lithium-ion batteries commonly used in mobile applications, one electrode is made of an alloy of lithium, cobalt and oxygen written colloquially as LCO. The other electrode is made of graphitic carbon, the kind of material you find in the lead of a pencil.

Now here’s the magic and beauty of operation. The lithium ions are present in a type of solution immersed between these two electrodes called electrolyte. When the battery is being charged, the lithium ions travel towards the carbon electrode, and physically enter the carbon matrix. The ions actually sit inside the carbon material. Think of swiss cheese and filling the empty spaces lithium ions. When the battery is discharged, the opposite happens and the lithium ions insert themselves inside the LCO electrode. Every time the ions go back and forth, energy is stored then returned. Very elegant. 

In real life, battery manufacturers build large sheets of electrodes, I mean very large, several feet wide and tens of yards in length, then assemble the sandwiched structure, then usually (though not always) roll it together in a cigar-like shape to make the device as compact as possible.

A photo showing the internal structure of a lithium-ion battery. Close examination shows the cigar-like roll containing the electrodes and the electrolyte. Courtesy of Dr. Venkat Srinivasan at the Lawrence Berkeley National Labs.

One last word on safety. Why are lithium-ion batteries prone to catching fire? It’s a complex process but let me explain with a simple example. Under some extreme conditions, say if we are carelessly charging the battery over its maximum allowed limits or creating a short-circuit by punching a nail into the battery, oxygen is released from the LCO electrode. But lithium is very reactive in the presence of oxygen and immediately catches fire. And lithium fire cannot be put out with water. Water actually makes the situation worse.

The good news is that lithium-ion batteries have gotten extremely safe over the past few years as battery manufacturers have made their designs less prone to these failure, and electronic protection systems ensure that the battery never sees extreme conditions.